How to enable GPU rendering on older Nvidia GPUs?
As iKlsR said in the comments to your question, we discarded the sm_13 cycles kernel... This is exactly the one needed for your series of cards. Please take my advice when I talk to you as a developer - you don't want to enable this for your card. 

First of all it's a hassle to setup nowadays! We don't care for this cards anymore, we don't even check if the code still compiles for it (and it certainly won't with our latest speed improvements for Cycles).
Second, it's slower than rendering with CPU in your case.
Third and foremost you are missing some important features with your kind of card:

There is no support for:


Render-passes, 
Ambient Occlusion,  
Many Cycles multi closures 
Hairs
Motion blur ...


As you can see, this is by no means the optimal way to go! I understand when someone doesn't want to upgrade a system, but in this case you should upgrade for sure!